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Effect of quench rate on the intrinsic coercivity of iron-rare earth-boron permanent magnets
OOOI-6160/X8 $3.00 f0.00 CopyrIght ~(‘81988 Pergamon Press plc
Acru rvrruli. Vol. 36. No. 12. pp. 3137-3147, 1988 Printed in Great Britain. All nghts reserved
EFFECT OF QUENCH RATE ON THE INTRINSIC COERCIVITY OF IRON-RARE EARTH-BORON PERMANENT MAGNETS R. RAMESH Department of Materials Materials and Chemical
and G. THOMAS
Science and Mineral Engineering and National Center for Electron Microscopy, Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, U.S.A.
(Rweived
I I Norembrr
1987; in rec:i,red /h-n
I6 March
1988)
Abstract-The effect of post-sintering cooling rate on the microstructure and magnetic properties of Fe-Nd-B based magnets has been studied. It has been shown that the intrinsic coercivity and energy product vary inversely as the cooling rate. In all cases, the magnet behaves as a nucleation controlled magnet characterized by a high initial permeability, although nucleation of reverse domains occurs at progressively lower fields as the cooling rate is increased. Samples quenched into water and oil exhibited extensive intergranular quench cracking. Scanning Auger elemental mapping as well as energy dispersive X-ray microanalysis showed the presence of oxygen, along with the rare earths, in the grain boundary phase. A qualitative argument for the influence of oxygen in reducing the magnetocrystalline anisotropy of the RE,Fe,,B phase is presented. Finally. the effect of cooling rate on the intrinstc coercivity and energy product can be understood in terms of the effect of local strains, at the grain boundaries and interfaces, on the magnetocrystalline anisotropy of the RE?Fe,,B phase. RksumGNous avons etudie I’effet de la vitesse de refroidissement apres frittage sur la microstructure et sur les proprittes magnttiques d’aimants a base de FeeNd~-B. La coercitivite intrinseque et le produit d’energie sont inversement proportionnels a la vitesse de refroidissement. Dans tous les cas. I’aimant se comporte comme un aimant control& par la germination, caracterise par une forte permtabilitt initiale, bien que la germination des domaines inverses ait lieu pour des champs progressivement moins eleveds lorsqu’on eleve la vitesse de refoidissment. Les echantillons trempes dans I’eau et dans I’huile presentent de nombreuses fissures intergranulaires de trempe. Le rep&age des elements par balayage Auger ainsi que la microanalyse en rayons X-par dispersion d’energie r&lent la presence d’oxygene et de terres rares dans la phase intergranulaire. Nous proposons un argument qualitatif en faveur de I’influence de I’oxygene sur la reduction de l’anisotropie magnetocristalline de la phase. RE, Fe,, B. Enfin, on peut comprendre l’effet de la vitesse de refroidissament sur la coercivite intrinsique et sur le produit d’energie en fonction de l’effet des deformations locales, au niveau des joints de grains et des interfaces, sur I’anisotropie magnetocristalline de la phase RE2 Fe,, B. Zusammenfassung-Der EinfluB der Abkiihlrate nach dem Sintern von FeeNd-B-Magneten auf Mikrostruktur und magnetische Eigenschaften wurde untersucht. Die intrinsische Koerzitivkraft und das Energieprodukt variieren invers mit der Abkiihlrate. In samtlichen Fallen verhalt sich der Magnet nukleationsgesteuert mit einer hohen Anfangspermeabilitat, obwohl die Bildung umgekehrter Dominen bei fortschreitend geringeren Feldern ablauft, wenn die Abkiihlrate erhoht wird. Proben, die in Wasser und iii abgeschreckt worden waren, wiesen ausgedehnte intergranulare RiBbildung durch Abschrecken auf. Aus Auger-Elementbildern und aus der energiedispersiven Rontgenanalyse ergab sich, daR Sauerstoff zusammen mit den seltenen Erden in der Korngrenzphase vorlag. Es wird ein qualitatives Argument fur den Einflul3 des Sauerstoffs auf die Reduzierung der magnetokristallinen Anisotropie der Phase RE, Fe,, B (RE: seltene Erde) dargelegt. Zum SchluD wird gezeigt, daB der Einflul3 der Abkiihlungsrate auf die intrinsische Koerzitivkraft und das Energieprodukt mit dem EinfluD lokaler Verzerrungen an Korngrenzen und Grenzflachen auf die magnetokristalline Anisotropie der Phase REZFe,,B verstanden werden kann.
INTRODUCTION Ever
since
the discovery
of the phase Ndz Fe,, B [I, 21,
there have been a large number of investigations [3] aimed at understanding the origin of the record high hard magnetic properties of this system and also the cause for the changes in the intrinsic coercivity with thermal treatments. The excellent permanent magnetic properties of this system are primarily due to the large saturation magnetization (1.61 T at room temperature) of Nd?Fe,,B coupled with a moderately 3137
high anisotropy field of 70 kOe at room temperature, although the Curie temperature (312°C) is low compared to the SmCo, magnets (830°C). Permanent magnets have been prepared by both the P/M sintering route [4] as well as by melt-spinning [5]. In this paper, magnets prepared by the former technique have been examined. These sintered magnets belong to the class of magnets termed as “nucleationcontrolled” magnets. In such magnets, the reversal of magnetization of each grain is accomplished by the
3138
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
Fig. 1. A bright field transmission electron micrograph depicting the general microstructure of sintered and water quenched Fe-Rare earth-Boron permanent magnets. Notice the cracking at the grain boundaries due to strain energy release by specimen thinning for TEM.
nucleation and movement of domain walls to reverse the magnetization of a fully saturated grain. In “nucleation-controlled” magnets, the nucleation field [6] (value of the total field at which reversal starts) is controlled predominantly by the nucleation step. In Fe-Nd-B magnets, the exact details of the nucleation process are still unclear, although it is generally accepted that nucleation in the bulk of the magnet occurs at internal interfaces such as grain boundaries or two-phase interfaces. Some suggested mechanisms include (i) the effect of quenching stresses; (ii) the presence of a soft magnetic bee phase at the grain boundaries [7]; (iii) the role of oxygen in providing regions of lowered anisotropy 181. However, no conclusive mechanism has been proposed so far that
I
I
I
PERMANENT
MAGNETS
will explain not only the changes in intrinsic coercivity with thermal treatments but also the fact that the highest intrinsic coercivity is much lower than the anisotropy field, HA of 70 kOe. Since the magnetic properties are extremely sensitive to changes in the microstructure, it is important to characterize the microstructure and correlate their changes to those of the magnetic properties. The microstructure of sintered magnets has been investigated in some detail in earlier papers [g-13] and has been summarized by Livingston f14]. As shown in the bright field tmnsmission electron micrograph Fig. 1, the microstructure consists of grains of almost defect free Nd*Fe,,B, at the triple grain junctions of which a rare-earth rich f.c.c. phase has been observed. Electron microscopy studies [9-l I] show that the spatial extent of this phase varies from a few microns at the triple junctions, to a few tens of nanometers at the two-grain boundaries. It was also shown that the f.c.c. structure arises due to the presence of oxygen in this phase [9]. The role of this phase, in the magnetization reversal process is still unclear. In addition, a Ndr ++Fe,B, phase has also been found as inde~ndent grains. This phase is paramagnetic at room temperature and is not considered to be a useful constituent. Likewise, due to improper processing, hexagonal rare earth oxides have also been identified [l 11. In Fig. 1, the grains have cracked apart during thin foil preparation, indicating the presence of strains localized at these boundaries in a water quenched magnet. Earlier investigations have focussed on the effect of post-sintering heat treatment upon the intrinsic coercivity and energy product, (BH),,, [4,15-191. In the as-water quenched condition, the intrinsic coare low. However, upon anercivity and (BH), nealing at 5~6~*C for one hour, the intrinsic coercivity is almost tripled along with a considerable increase in the (BH),,, . This is clearly shown in Fig.
I
I
I
I
I
I
I
I
I
Woter Quenched Somple Fe-32.5Y0
RE-
Anneolina Time:
I
f
RT
I
I% B I hr
I
200 Annealing
400 Temperoture
600
I
8C
(“Cl
Fig. 2. Effect of isochronal post-sintering annealing on the intrinsic coercivity of a typical water quenched sintered F+Nd-B magnet.
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
2 which shows the intrinsic coercivity as a function of annealing temperature for a typical magnet which has been water quenched. It has also been shown that a high intrinsic coercivity can be obtained by slow cooling from the sintering temperature to room temperature [20]. With this as background, the present set of experiments were undertaken to examine the effect of post-sintering cooling rate upon the magnetic properties and microstructure of a typical sintered magnet. The rationale behind such a set of experiments was that if quenching stresses do indeed influence the intrinsic coercivity, then the cooling rate from the sintering temperature would have a significant influence upon the intrinsic coercivity. Also if the quenching process alters the microstructure significantly then the magnetic properties should also be influenced. Thus the microstructure of the magnets with different quenching rates was also examined and characterized employing various analytical techniques. EXPERIMENTAL The nominal alloy composition studied was Fe-32.5 wt% rare earth-l wt% B wherein the rare earth composition was 5% Ce-15% Pr-80% Nd. The magnets were prepared by a standard powder metallurgical technique [2,4]. In the case of the quench rate experiments, the samples were quenched from the sintering temperature into various media such as water, silicone oil, flowing Argon etc. to obtain different cooling rates.
Magnetic measurements were carried out on cylindrical samples (1 cm x 1 cm) using a hysteresigraph. A maximum field of 20 kG was applied to study the hysteresis loops as well as the nature of the virgin curve. All measurements were carried out at room temperature. The samples were characterized using a variety of techniques. Scanning electron microscopy of the samples was carried out to examine the effect of various cooling rates upon the structural integrity of the magnets, e.g. the presence of microcracks on the surfaces. Samples for Transmission Electron Microscopy (TEM) were prepared by sectioning a thin wafer from the bulk magnet, mechanical thinning and finally Argon ion milling to electron transparency at 6 kV with a beam current of 0.3-0.4 mA, corresponding to a specimen current of 30-40 @A. Conventional imaging and Analytical Electron Microscopy (AEM) were carried out in a Philips 400 TEM/STEM at 100 kV. Detection of oxygen was accomplished using an ultra-thin window detector on a 200 kV JOEL microscope at the National Center for Electron Microscopy. Care was taken to ensure statistical significance of the X-ray microanalytical data by acquiring at least 10’ counts. Spectral d~onvolution and qu~intification was carried out using theoretically
PERMANENT
3139
MAGNETS
generated k-factors. However, in the case of oxygen experimentally corrected values of k-factorswere used, Microanalysis line profiles were carried out at various regions in many samples in order to obtain a reasonable degree of reliability of the data. High resolution TEM was carried out on a JEOL 200CX at 200 kV. Auger electron spectroscopy (AES) and elemental mapping was carried out using a Phi Scanning Auger Microscope (SAM). Standard samples were prepared and were fractured in situ in a vacuum of 1O-9 torr. Both point analyses as well as elemental mapping were carried out. In addition, depth profiling was also carried out to study the composition as a function of depth. RESULTS Figure 3 shows the intrinsic coercivity and (BH),,, as a function of cooling rate. The vafues of cooling rates were determined from parallel experiments on alloy steels of composition Fe-10 wt% Cr-0.2 wt% C-l wt% Mn. Samples of the same size as the magnets, i.e. I cm x 1 cm cylinders were used. The cooling rates were measured by welding Pt-Rh thermocouples onto the surface of the sample and measuring the time-temperature profile. The cooling rate was determined from this profile. The maximum value of cooling rate was used. The cooling rates measured for air cooling, oil and water quenching were used as indicators of the quenching potential of the media, since no data are available on the thermal conductivity of sintered Fe-Nd-B magnets. In the case of the furnace cooled sample the cooling rate was 50”C/h, since this was carried out in a computerized furnace with a constant cooling rate. As is evident from Fig. 3, the cooling rate from the sintering temperature affects the microstructure sensitive magnetic properties. The remanence, however, is independent of the cooling rate, as would be expected, since it is fixed by the starting allow composition. This strong dependence of intrinsic coercivity and (BH),,, on the cooling rate indicates that quenching stresses, which also increase with increasing cooling rate [21], may ,50
10
0
y-
5Ce
~\
2 8
-40
\
30
3 2"
iz 2o
f B
10
:01
lo-
-JO 1 10-l Cooling rate
10
102
IO’
fT/sec)
Fig. 3. Effect of post-sjntering cooling rate on the intrinsic coercivity and energy product of Nd-Pr-Ce-Fe- B magnet.
3140
RAMESH
and THOMAS:
kin.
FURNACE COOLED (5o"c/souR)
z
IRON-RARE
9.15
kOe
iHC
=
Br
= 10.64kG
EARTH-BORON
PERMANENT
MAGNETS
AIR COOLED iHc = 4.71 kOs Br = 11.09kG
OIL QUENCHED
WATER QUENCHED iHC = 3.35 kOs Br = 10.82kG
Fig. 4. Virgin
curve
and corresponding hysteresis loop of Fe-Nd-Pr-Ce-B magnet media. The maximum applied fiend in each case was 25 kG.
play a significant role in aiding nucleation of reverse domains at the grain boundaries and/or two phase interfaces. The hysteresis curve and virgin magnetization plot for each of the cooling rates is shown in Fig. 4 (ad). The steep rise in the virgin curve, in all cases, indicates that the domain walls in the magnet can move very easily under small applied fields. Although the shape of the hysteresis loop still remains the same, nucleation is occurring at progressively lower fields as the cooling rate is increased. Figure S(a, b) show SEM images of the surface of the water cooled sample and the furnace cooled sample. The quench cracks on the ‘surface of the water quenched sample are clearly visible. Thus, the cracks in Fig. I are also probably due to quenching stresses. No such cracks were observed on the furnace cooled sample. This again supports the idea that quenching stresses can be significant in explaining the lowering of intrinsic coercivity with increased cooling rate. The fracture mode in the water quenched sample was examined by carefully opening up one of the cracks on the surface and examining it immediately under the SEM. The fractograph in Fig. 6 shows almost completely intergranular fracture. These results show the weakness of the grain boundaries. Similar observations have been made by Doser and Smeggil [22] in the case of Sm-Co alloys. The fracture surface of the in situ fracture sample of a notched water quenched sample in the SAM was examined for the presence of various elements. Primarily, the surface was examined for the presence of oxygen, since in an earlier paper it was reported that in the slowly cooled condition, the grain boundaries contained significant amounts of oxygen [9]. The fracture surface, shown in Fig 7(a) is predominantly
cooled
in different
intergranular, although transgranular cleavage regions can be clearly recognized. It must be pointed out that this is not contradictory to the result of Fig.
Fig. 5. (a) Scanning electron micrograph of the polished surface of a water quenched sample showing macroscopic quench cracks; (b) scanning electron micrograph of the polished surface of a furnace cooled sample (WC/h). In both cases the grain size is about 1Opm so the grains themselves are not visible.
RAMESH and THOMAS:
IRON RARE EARTH-BORON
PERMANENT
MAGNETS
3141
the fracture is due to quenching stresses, while in this case, the origin of the stresses is due to the notch in the sample (the stress concentrator to enable fracture). Also note that transgranular fracture occurs only in regions of large grains (as shown by the arrows). Figure g(a) shows an Auger spectrum from the transgranular region, while the peak to peak ratios as a function of sputtering time are shown in Fig. 8(b). Figure 9(a, b) show the corresponding plots for the intergranular facet. The main point to be noted in Fig. 9(a) is that there is a large oxygen edge in the intergranular region. This is not superficially adsorbed oxygen, since the depth profile indicates a significant peak to peak ratio for sputtering times of up to 5min. On the other hand, the t~nsgranular region exhibits only superficially adsorbed oxygen to the extent of about 60 s sputtering time (equivalent to about 3OA). The oxygen and Nd Auger electron maps are shown in Fig 7(b, c) and clearly show that oxygen co-segregates with Nd in the grain boundary regions. Thus, it is possible that the predominantly intergranular fracture observed can be caused by oxygen embrittlement possibly associated with the rare earth elements as oxides. Similar arguments have been used by Westbook and Co-workers [23,24] for intermetallic compounds. Some of the microstructural details of the sintered magnets have been already discussed in earlier papers [7-121. With the use of an analytical electron microscope fitted with an ultra-thin window Energy Dispersive X-ray (EDX) detector, it was possible to detect oxygen in the grain boundary Nd-rich phase, thus independently verifying the SAM results. Figure IO(a) shows a typical EDX spectrum from the grain boundary phase. The low energy end is shown in Fig. 10(b), for a water quenched sample, wherein the oxygen peak can be clearly observed. Thus, along with the Auger data, it is now established that oxygen is an integral part of the grain boundary phase, both in the water quenched sample as well as in the slowly cooled sample 191.Microanalysis line profiles for Ce, Pr, Nd and Fe did not show any significant difference between the water quenched sample and the slow cooled sample. Figure 11(a) shows a typical line profile across the matrix-triple junction f.c.c. phase interface, in the case of a slowly cooled sample, while Fig. 11(b) shows the same for a water quenched sample. Although the f.c.c. phase is enriched in Ce and Pr, there is no significant difference between the two samples. Samples quenched into oil, or air cooled did not show any different microanalytical behaviour with respect to Ce, Pr and Nd. 6 where
Fig;. 6. Low magni~cation scanning electron micrograph of the fracture surface of the water quenched sample after the fraczture surface was opened, showing a predominantly inter-granuar fracture.
DISCUSSION
,. 7. (a) Fracture surface of in situ fractured Auger nple; fb) Oxygen Auger electron map of the region in (a); (c) Nd-Auger electron map of the region in (a). AM
/ Xl?-F
The actual cause of magnetization reversal in this class of magnets is still unclear, although the overall behaviour is consistent with that of a nucleation controlled magnet. In a recent paper (81, it was indicated that oxygen in the grain boundary phase
3142
RAMESH
and
THOMAS:
IRON-RARE
EARTH-BORON
PERMANENT
MAGNETS
(a) AES rurwy
SF=l.936,0.936
DAT=4.96
PT # =3
12/13/95
Y
7 Fe-Nd-8
Tranrgraln
area
6
3
G1 * iLi
4
z z 0 *
3
ZJ 2
1
0 300
0
400
500
600
700
600
900
1000
Kinetic energy (eV)
lb) AES
arofile
PT*C :2
12/13/66
Fe-Nd-B
SO6KG
transgrain
tracturr
FE1
1
2
3
4
5
Sputter time (mirl) Fig. 8. (a) Auger spectrum from transgranular peak-to-peak ratios as a function of sputtering
fracture region [region A in Fig. 7(a)]; (b) 0, Fe and Nd time. Note the sharp drop in the oxygen peak-to-peak ratio
after about 60 s of milling.
could play a significant role in reducing the magnetocrystalline anisotropy close to the interfaces. Another proposed mechanism attributed the nucleation of reverse domains to the presence of a soft magnetic b.c.c. phase at the gain boundaries [7]. However, this
observation has been shown to be due to an artefact of Argon ion beam specimen preparation [9]. Figure 12 is a typical high resolution lattice fringe image, in the case of a water quenched sample, where the interface between the matrix RE2 Fe,, B phase and the
RAMESH
and THOMAS:
IRON-RARE
EARTH-BORON
sF’3.8?7,3.616 Fe-Nd-s
p&T=4.@6
paint t) 1 Intargranular
PT#
400
500
Kinetic
AES
profile
Pt # = 1
12/13185
Fs-Nd-8
=t
r21%3R65
MAGNETS
3143
Y
rogian
Fe
300
PERMANENT
energy
Fs
600
Nd
Fe Nd
700
a00
900
1000
(eV)
S0sKO
Intrrgranular
fracturr
6
0
1
2 Sputter
3
time
4
5
(min)
Fig. 9. ia) Auger spectrum from inter-granular fracture region [region @) in Fig. ?(a)]; (b) 0, Fe and Nd function of milling time. Note the s&r&ant oxygen peak-to-peak ratio even after five minute of milling.
peak-to-peak ratitss as a
f.c.c. phase has been imaged. Clearly, no third phase is observed between the matrix phase and the f.c.c. phase, but the characterization of possible quench induced strains requires more detailed analyses, especially in thick foils. grain buundary
The role of oxygen and that of quench rate are fikely ta be synergistic in nature. However, even in samples which are cooled very slowly (.W’C/h) the intrinsic coercivity (9.8 kUe) is much below the anisotropy field (-30 kOe at room temperature for
3144
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
PERMANENT
MAGNETS
Ia) 3CEl
F.C.C.
Vert =
10,000
boundory
phose
counts
Preset
= 100
Elapsed
Range
= 10.230
keV
F.C.C.
Groin
boundory
phase
1000
counts
0.000
1-
Groin
s
=300
10.110
s
-)
.b) 3CEl Vert
=
Disp = 1
Preset Elopsed
= 100
s
= 300s
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0.200
Ronge
= 10.230
keV
1.460
-+
Fig. 10. (a) A typical EDX spectrum from the grain boundary phase. obtained using an ultra-thin window (UTW) EDX detector; (b) the low energy end of the spectrum showing the oxygen peak clearly.
NdzFe,,B), indicating that easy wall nucleation occurs at sites of considerably lowered magnetocrystalline anisotropy or higher demagnetizing fields. It is in this context that the role of oxygen has to be examined in detail. Thus, in what follows, a qualitative argument will be presented to explain the role of oxygen in aiding wall nucleation, Consider a grain of Nd*Fe,,B in an aligned polycrystalline magnet. The magnetization reversal of this grain will be governed primarily by the defect configuration on its surface and the demagnetizing field. In this analysis, oxygen at the grain boundaries and in the f.c.c. phase/matrix phase interface is considered as one of the defects. Consider, specifically, a unit cell of the Nd,Fe,,B phase [1], shown in Fig. 13, at the surface of the grain. The environment of this unit cell is different in different directions. The Nd atoms of this unit cell will be in
close contact with the oxygen atoms in the grain boundary phase. Let an oxygen atom be located at an arbitrary postion, close to the Nd atom in this unit cell, as dipicted in Fig. 13. Due to the large electronegativity of oxygen, there will be a definite tendency for the oxygen atoms to attract electrons from the Nd atoms. Since at least 2/3 of the magnetocrystalline anisotropy of the Nd,Fe,,B phase comes from the Nd sub-lattice, any changes in the electronic structure of the Nd atom in the unit cell will bring about large changes in the magnetocrystalline anisotropy of the crystal. This effect of oxygen can be visualized to occur in another way. The Nd atom could be visualized as being oxidized by the oxygen atom, thus, effectively removing it from the unit cell. This could then be the cause for the drop in magnetocrystalline anisotropy. A similar mechanism has been used by Jin et al [25] who have considered the magneto-
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
(al Furnace cooled sample s
100
f
a0
g-
60
t *g
40
;i
0
20
Position
(pm)
(b) Water quenched sample
Position
(pm)
Fig. Il. (a) A typical line profile across the matrix-triple junction f.c.c. phase interface in a slow cooled sample; (b) a similar profile in the case of a water quenched sample. anisotropy of Sm i-3 near point defects in SmCo, and in Sm,Co,,. Their results indicate that in some cases, significantly lowered anisotropy could be observed for defect structures. This combined with the local demagnetizing fields, will lead to easy domain wali nucleation. The post-sintering quench rate has a definite influence upon the intrinsic coercivity, the B-H loop and thus on the energy product. Since the intrinsic coercivity decrease to a value much lower than the slowly cooled sample, some other mechanism, in addition to the effect of oxygen, must be envisaged. The SEM studies of the quenched samples clearly show that on increasing the cooling rate to above that of an oil quench, quench cracks proliferate the magcrystalline
Fig. 12. High resolution lattice fringe image of the interface between the matrix and the grain boundary f.c.c. phase in a water quenched sample. The interface does not contain a b.c.c. phase.
PERMANENT
MAGNETS
314s
net. The presence of these cracks suggest that the stresses (at least locally) are well above the fracture stress of the NdzFe,,B phase, or above the failure stress of the interphase interface. However, the observation of a predominantly intergranular fracture (Figs 1 and 6) indicates that the second case is more likely. In addition, the results of Auger elemental mapping as well as X-ray microanalysis indicate that the intergranular fracture is likely to be aided by the presence of oxygen in the grain boundary phase. In general, the intrinsic coercivity, iHc, for a nucIeation type magnet can be written as: iHc = c. HA-Nd.M, [26], where, HA is the anisotropy field of the main magnetic phase; c is a constant which depends upon the nature of the nucleation sites; N,, is the demagnetization constant which is dependent upon the shape of the grains and the degree of interaction across grain boundaries and two-phase interfaces; M, is the saturation magnetization. However, the anisotropy field will be strongly influenced by local changes in composition, local lattice strains, especially near grain boundaries and two-phase interfaces. Thus, an effective value of the anisotropy field must be used in the determinatjon of the intrinsic coercivity. These considerations show that two properties, i.e. the effective anisotropy field and the saturation magnetization will influence the value of intrinsic coercivity, for given values of “c” and Nd, If the quenching stresses are sufficient to cause cracking or microcracking of the polycrystalline magnet, then it is possible that the demagnetizing fields at the cracked surfaces will be important. This is because cracking causes the formation of free surfaces, at which free poles will arise. These free poles will lead to high local demagnetizing fields. However, the effect of such a phenomenon appears to be insignificant in the case of water quenched samples. This can be shown from the results of two experiments. In the first, a strong effect of post-quench anneal on the intrinsic coercivity is observed experimentally (as was shown in Fig. 2). Thus, if the sample is annealed at 50&6OO’C for one hour the iHc can be raised to almost that of the slow-cooled sample. However, if quench crack induced demagnetizing fields are the main cause for the drop in iHc, then the annealing treatment should not show an increase in iHc, since many cracks should still remain on annealing. The second evidence which rules out the idea that internal quench cracks induced demagnetizing fields are the cause for the drop in iHc is the results of applied field dependence of iHc. In general, for nucleation controlled magnets, a strong dependence of iHc upon the applied external field is observed [27]. The effective demagnetizing field can be determined from a plot of intrinsic coercivity vs applied field, HdPP.Figure 14 shows iHc vs HaPPfor: (1) the water quenched sample; (2) the water quenched and annealed sample; (3) the slowly cooled sample. There is no significant difference between the three samples in terms of the value of
3146
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
PERMANENT
MAGNETS
OXYGEN IN FCC PHASE
OXYGEN IN FCC PHASE
OXYGEN IN FCC PHASE
lF~EOF~~()F~i,~Fei28Fek~~F~k2~B~ GRAIN BOUNDARY MATRIX RE2Fe140
PHASE
4JLL
FCC PHASE
INTERFACE
Fig. 13. Schematic representation of a unit cell of the RE,Fe,,B phase, situated at the grain boundary or the two-phase interface of a grain. For x > 0, the environment consists of the oxygen stabilized f.c.c. phase, while for x < 0 it is the RE,Fe,,B phase. An oxygen atom at the interface is shown in order to depict possible interaction with Nd in the matrix phase.
Harp at which iHc begins to rise (the effective demagnetizing field). Thus, the effect of demagnetizing field due to internal quench cracks does not appear to be the dominant cause for the drop in iHc with cooling rate, although quench cracks are observed. It is clear that the effective anisotropy field near the interfaces must be examined in detail. Specifically, the effect of local composition, especially oxygen, and the effect of localized strains, primarily due to quenching, on the magnetocrystalline anisotropy and the effective anisotropy field need further examination. Theoretical and experimental estimations of these effects are currently under way.
the differently treated magnets, it is shown that demagnetization fields at quench cracks are not the main cause for the difference in intrinsic coercivity for the different cooling rates. A qualitative argument for the influence of oxygen in reducing the magnetocrystalline anisotropy field of the RE,Fe,,B phase has been suggested to explain the low value of
CONCLUSIONS
In conclusion it has been shown that post-sintering cooling rates have a definite influence on the intrinsic coercivity and energy product of Iron-Rare Earth-Boron magnets. After water quenching, the intrinsic coercivity was found to be lower than in a slowly cooled magnet. Concurrently, water quenched samples contained many intergranular quench that quenching stresses are cracks, indicating significant. No specific differences in microanalytical details of the rare earth elements in the matrix and grain boundary phase and their interface were observed for the different cooling rate samples. From the similar field dependence of intrinsic coercivity in
0 0
10
Applied
20
field,Hm,
(kG)
Fig. 14. Applied field dependence of the intrinsic coercivity for (1) water quenched samples; (2) water quenched + annealed samples; (3) slowly cooled samples.
RAMESH and THOMAS:
IRON-RARE EARTH-BORON PERMANENT MAGNETS
intrinsic coercivity compared to the anisotropy field, even in a slowly cooled sample. Finally, it is possible that local strains at the grain boundaries and twophase interfaces could cause a large reduction in the magnetocrystalline anisotropy of the RE,Fe,,B phase, thus leading to an even lower intrinsic coercivity in the rapidly quenched magnet. Ackn~wledgeme~f.~-The
authors wish to thank Professor
M. Okada, Tohuku University, Japan for kindty providing the alloys used in this study. The authors also wish to acknowledge Professor W. A. Soffa for stimulating discussions on this subject. This work was supported by the Director, Office of Basic Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under contract No. DE-AC0376SF00098. REFERENCES 1. J. F. Herbst, J. 3. Croat, F. E.(1984). Pinkerton and W. B. Yelon, Phys. Rev. B. 29, 4176 2. M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura and K. Hiraga, IEEE Trans. h4ag. Mag-20,1584 (1984). 3. See for example Proc. Znfermag 1984, 1985, 1986 and 1987; Proc. Annuaf EMU
Cor$
9. R. Ramesh, J. K. Chen and G. Thomas, J. appl. Phys. 61, 2993 (1987). 10. R. K. Mishra, J. K. Chen and G. Thomas, J. appl. Phys. 59, 2244 (1986).
11. R. Ramesh, K. M. Krishnan, E. Goo, G. Thomas, M. Okada and M. Homma, J. Magnm and Mug. Mater. 54-57,
563 (1986).
12. M. Tokunaga, M. Tobise, N. Meguro and H. Harada, IEEE Trans. Mag. Mag-22, 904 (1986).
13. P. Schrey, IEEE Trans. Mag. Mag-22, 913 (1986). 14. J. D. Livingston, Proc. ASM Symp. on SoJf and Hard Magnetic Maferja~s (edited by J. A. Salsgiver et al.) (1986). 15. M. Tokunaga, N. Meguro, M. Endoh, S. Tanigawa and H. Harada, IEEE Trans. Mug. Mag-21, 1964 (1985). 16. T. Bailey and 1. R. Harris, J. Mufer. Sci. Lett. 4, 151 (1985). 17. S. 2. Zhou, L. Li, L. D. Zhang and Q. Hu, J. Magnm Mag. Mater. 54-57, (1986).
18. S. E. Hsu, K. L. Wang and L. C. Su, Paper No. CC-04, Intermag 1987, Tokyo, Japan. 19. J. Ormeiod, J. less-common Metals 111, 49 (1985). 20. M. Okada. S. Sueimoto. C. Ishizaka. T. Tanaka and M. Homma, j. a~& Phys.‘57, 4i46 (1985). H. Hencke, J. R. Thomas Jr and D. P. H. Hasselmann, 21’ 22,
1984, 198.5, 1986.
4. M. Sagawa. S. Fuiimura, N. Togawa, H. Yamamoto and Yr Maisuura, j. appl. Phys. 35, 2083 (1984). 5. R. W. Lee, E. G. Brewer and N. A. Schaffel, IEEE Trans. Mag. Mag-21 1958 (1985). 6. J. D. Livingston, Paper No. AC-06, Intermag-1987, Tokyo, Japan. 7. K. I%ragai M., Hirabayashi, M. Sagawa and Y. Matsuura. Jao. J. anoi. Phvs. 24. L30 11985): K. Hiraaa. M. Hiradayashi, M.-Saga&a a&l Y. katsuira, Jap. i appl. Phys. 24, 699 (1985). 8. R. Ramesh, G. Thomas, M. Okada and M. Homma, Paper No. AC-07, Intermag 1987, Tokyo, Japan.
3147
_
J. Am. Ceram. Sac. 67, No. 6, 393 (1984). M. Doser and J. G. Smeggil, IEEE Trans. Mugn.
Meg-9, 168 (1973).
LJ. J. H, Westbrook and D. L. Wood, J. fnst Mefafs 91, 174 24,
(1962). A. U. Seybolt and J. H. Westbrook, Acta metall. 12,449 (1964).
25. H. M. Jin, H. N. Chen, D. S. Tang, J. F. Han and Y. Shi, Prac. Sixth Int. Workshop on Rare Earth Permanent Magnets and their Applications (Edited by J. Fidler), p. 549 (1982). 26 G. Herzer, W. Fernengel and E. Adler, J. Magnm Mug. 27,
Mater. 58, 48 (1986).
J. J. Becker, J. appl. Phjps. 39, 1270 (1968).
Acru rvrruli. Vol. 36. No. 12. pp. 3137-3147, 1988 Printed in Great Britain. All nghts reserved
EFFECT OF QUENCH RATE ON THE INTRINSIC COERCIVITY OF IRON-RARE EARTH-BORON PERMANENT MAGNETS R. RAMESH Department of Materials Materials and Chemical
and G. THOMAS
Science and Mineral Engineering and National Center for Electron Microscopy, Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, U.S.A.
(Rweived
I I Norembrr
1987; in rec:i,red /h-n
I6 March
1988)
Abstract-The effect of post-sintering cooling rate on the microstructure and magnetic properties of Fe-Nd-B based magnets has been studied. It has been shown that the intrinsic coercivity and energy product vary inversely as the cooling rate. In all cases, the magnet behaves as a nucleation controlled magnet characterized by a high initial permeability, although nucleation of reverse domains occurs at progressively lower fields as the cooling rate is increased. Samples quenched into water and oil exhibited extensive intergranular quench cracking. Scanning Auger elemental mapping as well as energy dispersive X-ray microanalysis showed the presence of oxygen, along with the rare earths, in the grain boundary phase. A qualitative argument for the influence of oxygen in reducing the magnetocrystalline anisotropy of the RE,Fe,,B phase is presented. Finally. the effect of cooling rate on the intrinstc coercivity and energy product can be understood in terms of the effect of local strains, at the grain boundaries and interfaces, on the magnetocrystalline anisotropy of the RE?Fe,,B phase. RksumGNous avons etudie I’effet de la vitesse de refroidissement apres frittage sur la microstructure et sur les proprittes magnttiques d’aimants a base de FeeNd~-B. La coercitivite intrinseque et le produit d’energie sont inversement proportionnels a la vitesse de refroidissement. Dans tous les cas. I’aimant se comporte comme un aimant control& par la germination, caracterise par une forte permtabilitt initiale, bien que la germination des domaines inverses ait lieu pour des champs progressivement moins eleveds lorsqu’on eleve la vitesse de refoidissment. Les echantillons trempes dans I’eau et dans I’huile presentent de nombreuses fissures intergranulaires de trempe. Le rep&age des elements par balayage Auger ainsi que la microanalyse en rayons X-par dispersion d’energie r&lent la presence d’oxygene et de terres rares dans la phase intergranulaire. Nous proposons un argument qualitatif en faveur de I’influence de I’oxygene sur la reduction de l’anisotropie magnetocristalline de la phase. RE, Fe,, B. Enfin, on peut comprendre l’effet de la vitesse de refroidissament sur la coercivite intrinsique et sur le produit d’energie en fonction de l’effet des deformations locales, au niveau des joints de grains et des interfaces, sur I’anisotropie magnetocristalline de la phase RE2 Fe,, B. Zusammenfassung-Der EinfluB der Abkiihlrate nach dem Sintern von FeeNd-B-Magneten auf Mikrostruktur und magnetische Eigenschaften wurde untersucht. Die intrinsische Koerzitivkraft und das Energieprodukt variieren invers mit der Abkiihlrate. In samtlichen Fallen verhalt sich der Magnet nukleationsgesteuert mit einer hohen Anfangspermeabilitat, obwohl die Bildung umgekehrter Dominen bei fortschreitend geringeren Feldern ablauft, wenn die Abkiihlrate erhoht wird. Proben, die in Wasser und iii abgeschreckt worden waren, wiesen ausgedehnte intergranulare RiBbildung durch Abschrecken auf. Aus Auger-Elementbildern und aus der energiedispersiven Rontgenanalyse ergab sich, daR Sauerstoff zusammen mit den seltenen Erden in der Korngrenzphase vorlag. Es wird ein qualitatives Argument fur den Einflul3 des Sauerstoffs auf die Reduzierung der magnetokristallinen Anisotropie der Phase RE, Fe,, B (RE: seltene Erde) dargelegt. Zum SchluD wird gezeigt, daB der Einflul3 der Abkiihlungsrate auf die intrinsische Koerzitivkraft und das Energieprodukt mit dem EinfluD lokaler Verzerrungen an Korngrenzen und Grenzflachen auf die magnetokristalline Anisotropie der Phase REZFe,,B verstanden werden kann.
INTRODUCTION Ever
since
the discovery
of the phase Ndz Fe,, B [I, 21,
there have been a large number of investigations [3] aimed at understanding the origin of the record high hard magnetic properties of this system and also the cause for the changes in the intrinsic coercivity with thermal treatments. The excellent permanent magnetic properties of this system are primarily due to the large saturation magnetization (1.61 T at room temperature) of Nd?Fe,,B coupled with a moderately 3137
high anisotropy field of 70 kOe at room temperature, although the Curie temperature (312°C) is low compared to the SmCo, magnets (830°C). Permanent magnets have been prepared by both the P/M sintering route [4] as well as by melt-spinning [5]. In this paper, magnets prepared by the former technique have been examined. These sintered magnets belong to the class of magnets termed as “nucleationcontrolled” magnets. In such magnets, the reversal of magnetization of each grain is accomplished by the
3138
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
Fig. 1. A bright field transmission electron micrograph depicting the general microstructure of sintered and water quenched Fe-Rare earth-Boron permanent magnets. Notice the cracking at the grain boundaries due to strain energy release by specimen thinning for TEM.
nucleation and movement of domain walls to reverse the magnetization of a fully saturated grain. In “nucleation-controlled” magnets, the nucleation field [6] (value of the total field at which reversal starts) is controlled predominantly by the nucleation step. In Fe-Nd-B magnets, the exact details of the nucleation process are still unclear, although it is generally accepted that nucleation in the bulk of the magnet occurs at internal interfaces such as grain boundaries or two-phase interfaces. Some suggested mechanisms include (i) the effect of quenching stresses; (ii) the presence of a soft magnetic bee phase at the grain boundaries [7]; (iii) the role of oxygen in providing regions of lowered anisotropy 181. However, no conclusive mechanism has been proposed so far that
I
I
I
PERMANENT
MAGNETS
will explain not only the changes in intrinsic coercivity with thermal treatments but also the fact that the highest intrinsic coercivity is much lower than the anisotropy field, HA of 70 kOe. Since the magnetic properties are extremely sensitive to changes in the microstructure, it is important to characterize the microstructure and correlate their changes to those of the magnetic properties. The microstructure of sintered magnets has been investigated in some detail in earlier papers [g-13] and has been summarized by Livingston f14]. As shown in the bright field tmnsmission electron micrograph Fig. 1, the microstructure consists of grains of almost defect free Nd*Fe,,B, at the triple grain junctions of which a rare-earth rich f.c.c. phase has been observed. Electron microscopy studies [9-l I] show that the spatial extent of this phase varies from a few microns at the triple junctions, to a few tens of nanometers at the two-grain boundaries. It was also shown that the f.c.c. structure arises due to the presence of oxygen in this phase [9]. The role of this phase, in the magnetization reversal process is still unclear. In addition, a Ndr ++Fe,B, phase has also been found as inde~ndent grains. This phase is paramagnetic at room temperature and is not considered to be a useful constituent. Likewise, due to improper processing, hexagonal rare earth oxides have also been identified [l 11. In Fig. 1, the grains have cracked apart during thin foil preparation, indicating the presence of strains localized at these boundaries in a water quenched magnet. Earlier investigations have focussed on the effect of post-sintering heat treatment upon the intrinsic coercivity and energy product, (BH),,, [4,15-191. In the as-water quenched condition, the intrinsic coare low. However, upon anercivity and (BH), nealing at 5~6~*C for one hour, the intrinsic coercivity is almost tripled along with a considerable increase in the (BH),,, . This is clearly shown in Fig.
I
I
I
I
I
I
I
I
I
Woter Quenched Somple Fe-32.5Y0
RE-
Anneolina Time:
I
f
RT
I
I% B I hr
I
200 Annealing
400 Temperoture
600
I
8C
(“Cl
Fig. 2. Effect of isochronal post-sintering annealing on the intrinsic coercivity of a typical water quenched sintered F+Nd-B magnet.
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
2 which shows the intrinsic coercivity as a function of annealing temperature for a typical magnet which has been water quenched. It has also been shown that a high intrinsic coercivity can be obtained by slow cooling from the sintering temperature to room temperature [20]. With this as background, the present set of experiments were undertaken to examine the effect of post-sintering cooling rate upon the magnetic properties and microstructure of a typical sintered magnet. The rationale behind such a set of experiments was that if quenching stresses do indeed influence the intrinsic coercivity, then the cooling rate from the sintering temperature would have a significant influence upon the intrinsic coercivity. Also if the quenching process alters the microstructure significantly then the magnetic properties should also be influenced. Thus the microstructure of the magnets with different quenching rates was also examined and characterized employing various analytical techniques. EXPERIMENTAL The nominal alloy composition studied was Fe-32.5 wt% rare earth-l wt% B wherein the rare earth composition was 5% Ce-15% Pr-80% Nd. The magnets were prepared by a standard powder metallurgical technique [2,4]. In the case of the quench rate experiments, the samples were quenched from the sintering temperature into various media such as water, silicone oil, flowing Argon etc. to obtain different cooling rates.
Magnetic measurements were carried out on cylindrical samples (1 cm x 1 cm) using a hysteresigraph. A maximum field of 20 kG was applied to study the hysteresis loops as well as the nature of the virgin curve. All measurements were carried out at room temperature. The samples were characterized using a variety of techniques. Scanning electron microscopy of the samples was carried out to examine the effect of various cooling rates upon the structural integrity of the magnets, e.g. the presence of microcracks on the surfaces. Samples for Transmission Electron Microscopy (TEM) were prepared by sectioning a thin wafer from the bulk magnet, mechanical thinning and finally Argon ion milling to electron transparency at 6 kV with a beam current of 0.3-0.4 mA, corresponding to a specimen current of 30-40 @A. Conventional imaging and Analytical Electron Microscopy (AEM) were carried out in a Philips 400 TEM/STEM at 100 kV. Detection of oxygen was accomplished using an ultra-thin window detector on a 200 kV JOEL microscope at the National Center for Electron Microscopy. Care was taken to ensure statistical significance of the X-ray microanalytical data by acquiring at least 10’ counts. Spectral d~onvolution and qu~intification was carried out using theoretically
PERMANENT
3139
MAGNETS
generated k-factors. However, in the case of oxygen experimentally corrected values of k-factorswere used, Microanalysis line profiles were carried out at various regions in many samples in order to obtain a reasonable degree of reliability of the data. High resolution TEM was carried out on a JEOL 200CX at 200 kV. Auger electron spectroscopy (AES) and elemental mapping was carried out using a Phi Scanning Auger Microscope (SAM). Standard samples were prepared and were fractured in situ in a vacuum of 1O-9 torr. Both point analyses as well as elemental mapping were carried out. In addition, depth profiling was also carried out to study the composition as a function of depth. RESULTS Figure 3 shows the intrinsic coercivity and (BH),,, as a function of cooling rate. The vafues of cooling rates were determined from parallel experiments on alloy steels of composition Fe-10 wt% Cr-0.2 wt% C-l wt% Mn. Samples of the same size as the magnets, i.e. I cm x 1 cm cylinders were used. The cooling rates were measured by welding Pt-Rh thermocouples onto the surface of the sample and measuring the time-temperature profile. The cooling rate was determined from this profile. The maximum value of cooling rate was used. The cooling rates measured for air cooling, oil and water quenching were used as indicators of the quenching potential of the media, since no data are available on the thermal conductivity of sintered Fe-Nd-B magnets. In the case of the furnace cooled sample the cooling rate was 50”C/h, since this was carried out in a computerized furnace with a constant cooling rate. As is evident from Fig. 3, the cooling rate from the sintering temperature affects the microstructure sensitive magnetic properties. The remanence, however, is independent of the cooling rate, as would be expected, since it is fixed by the starting allow composition. This strong dependence of intrinsic coercivity and (BH),,, on the cooling rate indicates that quenching stresses, which also increase with increasing cooling rate [21], may ,50
10
0
y-
5Ce
~\
2 8
-40
\
30
3 2"
iz 2o
f B
10
:01
lo-
-JO 1 10-l Cooling rate
10
102
IO’
fT/sec)
Fig. 3. Effect of post-sjntering cooling rate on the intrinsic coercivity and energy product of Nd-Pr-Ce-Fe- B magnet.
3140
RAMESH
and THOMAS:
kin.
FURNACE COOLED (5o"c/souR)
z
IRON-RARE
9.15
kOe
iHC
=
Br
= 10.64kG
EARTH-BORON
PERMANENT
MAGNETS
AIR COOLED iHc = 4.71 kOs Br = 11.09kG
OIL QUENCHED
WATER QUENCHED iHC = 3.35 kOs Br = 10.82kG
Fig. 4. Virgin
curve
and corresponding hysteresis loop of Fe-Nd-Pr-Ce-B magnet media. The maximum applied fiend in each case was 25 kG.
play a significant role in aiding nucleation of reverse domains at the grain boundaries and/or two phase interfaces. The hysteresis curve and virgin magnetization plot for each of the cooling rates is shown in Fig. 4 (ad). The steep rise in the virgin curve, in all cases, indicates that the domain walls in the magnet can move very easily under small applied fields. Although the shape of the hysteresis loop still remains the same, nucleation is occurring at progressively lower fields as the cooling rate is increased. Figure S(a, b) show SEM images of the surface of the water cooled sample and the furnace cooled sample. The quench cracks on the ‘surface of the water quenched sample are clearly visible. Thus, the cracks in Fig. I are also probably due to quenching stresses. No such cracks were observed on the furnace cooled sample. This again supports the idea that quenching stresses can be significant in explaining the lowering of intrinsic coercivity with increased cooling rate. The fracture mode in the water quenched sample was examined by carefully opening up one of the cracks on the surface and examining it immediately under the SEM. The fractograph in Fig. 6 shows almost completely intergranular fracture. These results show the weakness of the grain boundaries. Similar observations have been made by Doser and Smeggil [22] in the case of Sm-Co alloys. The fracture surface of the in situ fracture sample of a notched water quenched sample in the SAM was examined for the presence of various elements. Primarily, the surface was examined for the presence of oxygen, since in an earlier paper it was reported that in the slowly cooled condition, the grain boundaries contained significant amounts of oxygen [9]. The fracture surface, shown in Fig 7(a) is predominantly
cooled
in different
intergranular, although transgranular cleavage regions can be clearly recognized. It must be pointed out that this is not contradictory to the result of Fig.
Fig. 5. (a) Scanning electron micrograph of the polished surface of a water quenched sample showing macroscopic quench cracks; (b) scanning electron micrograph of the polished surface of a furnace cooled sample (WC/h). In both cases the grain size is about 1Opm so the grains themselves are not visible.
RAMESH and THOMAS:
IRON RARE EARTH-BORON
PERMANENT
MAGNETS
3141
the fracture is due to quenching stresses, while in this case, the origin of the stresses is due to the notch in the sample (the stress concentrator to enable fracture). Also note that transgranular fracture occurs only in regions of large grains (as shown by the arrows). Figure g(a) shows an Auger spectrum from the transgranular region, while the peak to peak ratios as a function of sputtering time are shown in Fig. 8(b). Figure 9(a, b) show the corresponding plots for the intergranular facet. The main point to be noted in Fig. 9(a) is that there is a large oxygen edge in the intergranular region. This is not superficially adsorbed oxygen, since the depth profile indicates a significant peak to peak ratio for sputtering times of up to 5min. On the other hand, the t~nsgranular region exhibits only superficially adsorbed oxygen to the extent of about 60 s sputtering time (equivalent to about 3OA). The oxygen and Nd Auger electron maps are shown in Fig 7(b, c) and clearly show that oxygen co-segregates with Nd in the grain boundary regions. Thus, it is possible that the predominantly intergranular fracture observed can be caused by oxygen embrittlement possibly associated with the rare earth elements as oxides. Similar arguments have been used by Westbook and Co-workers [23,24] for intermetallic compounds. Some of the microstructural details of the sintered magnets have been already discussed in earlier papers [7-121. With the use of an analytical electron microscope fitted with an ultra-thin window Energy Dispersive X-ray (EDX) detector, it was possible to detect oxygen in the grain boundary Nd-rich phase, thus independently verifying the SAM results. Figure IO(a) shows a typical EDX spectrum from the grain boundary phase. The low energy end is shown in Fig. 10(b), for a water quenched sample, wherein the oxygen peak can be clearly observed. Thus, along with the Auger data, it is now established that oxygen is an integral part of the grain boundary phase, both in the water quenched sample as well as in the slowly cooled sample 191.Microanalysis line profiles for Ce, Pr, Nd and Fe did not show any significant difference between the water quenched sample and the slow cooled sample. Figure 11(a) shows a typical line profile across the matrix-triple junction f.c.c. phase interface, in the case of a slowly cooled sample, while Fig. 11(b) shows the same for a water quenched sample. Although the f.c.c. phase is enriched in Ce and Pr, there is no significant difference between the two samples. Samples quenched into oil, or air cooled did not show any different microanalytical behaviour with respect to Ce, Pr and Nd. 6 where
Fig;. 6. Low magni~cation scanning electron micrograph of the fracture surface of the water quenched sample after the fraczture surface was opened, showing a predominantly inter-granuar fracture.
DISCUSSION
,. 7. (a) Fracture surface of in situ fractured Auger nple; fb) Oxygen Auger electron map of the region in (a); (c) Nd-Auger electron map of the region in (a). AM
/ Xl?-F
The actual cause of magnetization reversal in this class of magnets is still unclear, although the overall behaviour is consistent with that of a nucleation controlled magnet. In a recent paper (81, it was indicated that oxygen in the grain boundary phase
3142
RAMESH
and
THOMAS:
IRON-RARE
EARTH-BORON
PERMANENT
MAGNETS
(a) AES rurwy
SF=l.936,0.936
DAT=4.96
PT # =3
12/13/95
Y
7 Fe-Nd-8
Tranrgraln
area
6
3
G1 * iLi
4
z z 0 *
3
ZJ 2
1
0 300
0
400
500
600
700
600
900
1000
Kinetic energy (eV)
lb) AES
arofile
PT*C :2
12/13/66
Fe-Nd-B
SO6KG
transgrain
tracturr
FE1
1
2
3
4
5
Sputter time (mirl) Fig. 8. (a) Auger spectrum from transgranular peak-to-peak ratios as a function of sputtering
fracture region [region A in Fig. 7(a)]; (b) 0, Fe and Nd time. Note the sharp drop in the oxygen peak-to-peak ratio
after about 60 s of milling.
could play a significant role in reducing the magnetocrystalline anisotropy close to the interfaces. Another proposed mechanism attributed the nucleation of reverse domains to the presence of a soft magnetic b.c.c. phase at the gain boundaries [7]. However, this
observation has been shown to be due to an artefact of Argon ion beam specimen preparation [9]. Figure 12 is a typical high resolution lattice fringe image, in the case of a water quenched sample, where the interface between the matrix RE2 Fe,, B phase and the
RAMESH
and THOMAS:
IRON-RARE
EARTH-BORON
sF’3.8?7,3.616 Fe-Nd-s
p&T=4.@6
paint t) 1 Intargranular
PT#
400
500
Kinetic
AES
profile
Pt # = 1
12/13185
Fs-Nd-8
=t
r21%3R65
MAGNETS
3143
Y
rogian
Fe
300
PERMANENT
energy
Fs
600
Nd
Fe Nd
700
a00
900
1000
(eV)
S0sKO
Intrrgranular
fracturr
6
0
1
2 Sputter
3
time
4
5
(min)
Fig. 9. ia) Auger spectrum from inter-granular fracture region [region @) in Fig. ?(a)]; (b) 0, Fe and Nd function of milling time. Note the s&r&ant oxygen peak-to-peak ratio even after five minute of milling.
peak-to-peak ratitss as a
f.c.c. phase has been imaged. Clearly, no third phase is observed between the matrix phase and the f.c.c. phase, but the characterization of possible quench induced strains requires more detailed analyses, especially in thick foils. grain buundary
The role of oxygen and that of quench rate are fikely ta be synergistic in nature. However, even in samples which are cooled very slowly (.W’C/h) the intrinsic coercivity (9.8 kUe) is much below the anisotropy field (-30 kOe at room temperature for
3144
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
PERMANENT
MAGNETS
Ia) 3CEl
F.C.C.
Vert =
10,000
boundory
phose
counts
Preset
= 100
Elapsed
Range
= 10.230
keV
F.C.C.
Groin
boundory
phase
1000
counts
0.000
1-
Groin
s
=300
10.110
s
-)
.b) 3CEl Vert
=
Disp = 1
Preset Elopsed
= 100
s
= 300s
j. ,, ,:j: .::: ,, j:.: .j’Y“: :: : :..:: :’ ‘: ‘, ..,: .:’: : .: ;, :.j,::j..:.::. ..‘. : :: ‘, .: .:” ...Y” ” : .: : j :’ :‘:: ..: : :. :: ::, ,A’: ”:: i::..:::: :::.:.::’ :’:’ .: ““” :: ‘: : .:. :j”: : :. : .L ..: : ” .: :.....: ” ” j:‘:’ii,; ;; :,:;j !,: j i ‘:’,:’I,,,;,, : ‘:’;’ ::::j‘:,,:;: : ;’ :: “:. 1,,j.: : :.,_i ,O.,( : ,. : : : i,. : : i. .!::’ : ,,: ::..:,...:. ” ..: .I‘. r. :.., :‘:j : ., ,. ,‘I.:: .., .... :: j:j.j::.. ,: :,,: ; :y::“::’ :,,,.” ‘1: j, ,, :‘: : 1: : A’ : : ,::: ::::.: .: ,:,, : .: ::“,.:,, ”: “’:. ; :: : ::.: :: : :, ‘:‘:.: !. ::,,,.,‘.: :.: : ,,,,: : : :: ,:. ,.:.:, : .‘. :.::,,:‘:‘: : :: ;” j : :: .: :’ ::. :: .:.::.: : :i ,: ,‘, ‘: I -_: .:.j,: ,tE: L I ;j :’ 1’NDM..:i,.,:y :,, jl : ; : : ,;y j. : .:::::..: ::‘:. .A:‘: ” ” . 1:.’ :‘:’ 1:j’ :’,,.,I: !. ,: ::‘: m’ ,, :: :,j.: .I :,. : .: .: .:.;j::: :’ : :I ., :.:: ” : -5 .. ‘.:..: : : ,, : .,,, .: :.:::;r‘PC “..” ‘: .i.l : :. ,j.im ~_: ,::, :, : :::..:.: ..,,,,, ‘. :::: .: ‘: .,, ,::j ‘. ..‘I ; :.: I s.‘: ...“’ .. I* : ‘. j ., ::j, .’ :.: .::,:: :,,j :..:: : : ,: ,,:: I’ : ‘..‘. -. : .j, :...: :j. :: i..: I ,,.::;:: : ” : :_ :. :’ .:‘: :. F ..i -.: j.:##.a -#“:I” i. .:. ‘: :. : .I .-:’ _..’ :.:I:,.j ,: .j:. ::. ,,..’: I ;?-. :, ,:,j:.I -._ .:_‘: ‘:: .:.: : _“, ,,:: .:6 .*._;._-.i._ :’ ., : :.: :: :,_A y& ;*” . ;- ~*+__w_.,: . :* 1-
0.200
Ronge
= 10.230
keV
1.460
-+
Fig. 10. (a) A typical EDX spectrum from the grain boundary phase. obtained using an ultra-thin window (UTW) EDX detector; (b) the low energy end of the spectrum showing the oxygen peak clearly.
NdzFe,,B), indicating that easy wall nucleation occurs at sites of considerably lowered magnetocrystalline anisotropy or higher demagnetizing fields. It is in this context that the role of oxygen has to be examined in detail. Thus, in what follows, a qualitative argument will be presented to explain the role of oxygen in aiding wall nucleation, Consider a grain of Nd*Fe,,B in an aligned polycrystalline magnet. The magnetization reversal of this grain will be governed primarily by the defect configuration on its surface and the demagnetizing field. In this analysis, oxygen at the grain boundaries and in the f.c.c. phase/matrix phase interface is considered as one of the defects. Consider, specifically, a unit cell of the Nd,Fe,,B phase [1], shown in Fig. 13, at the surface of the grain. The environment of this unit cell is different in different directions. The Nd atoms of this unit cell will be in
close contact with the oxygen atoms in the grain boundary phase. Let an oxygen atom be located at an arbitrary postion, close to the Nd atom in this unit cell, as dipicted in Fig. 13. Due to the large electronegativity of oxygen, there will be a definite tendency for the oxygen atoms to attract electrons from the Nd atoms. Since at least 2/3 of the magnetocrystalline anisotropy of the Nd,Fe,,B phase comes from the Nd sub-lattice, any changes in the electronic structure of the Nd atom in the unit cell will bring about large changes in the magnetocrystalline anisotropy of the crystal. This effect of oxygen can be visualized to occur in another way. The Nd atom could be visualized as being oxidized by the oxygen atom, thus, effectively removing it from the unit cell. This could then be the cause for the drop in magnetocrystalline anisotropy. A similar mechanism has been used by Jin et al [25] who have considered the magneto-
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
(al Furnace cooled sample s
100
f
a0
g-
60
t *g
40
;i
0
20
Position
(pm)
(b) Water quenched sample
Position
(pm)
Fig. Il. (a) A typical line profile across the matrix-triple junction f.c.c. phase interface in a slow cooled sample; (b) a similar profile in the case of a water quenched sample. anisotropy of Sm i-3 near point defects in SmCo, and in Sm,Co,,. Their results indicate that in some cases, significantly lowered anisotropy could be observed for defect structures. This combined with the local demagnetizing fields, will lead to easy domain wali nucleation. The post-sintering quench rate has a definite influence upon the intrinsic coercivity, the B-H loop and thus on the energy product. Since the intrinsic coercivity decrease to a value much lower than the slowly cooled sample, some other mechanism, in addition to the effect of oxygen, must be envisaged. The SEM studies of the quenched samples clearly show that on increasing the cooling rate to above that of an oil quench, quench cracks proliferate the magcrystalline
Fig. 12. High resolution lattice fringe image of the interface between the matrix and the grain boundary f.c.c. phase in a water quenched sample. The interface does not contain a b.c.c. phase.
PERMANENT
MAGNETS
314s
net. The presence of these cracks suggest that the stresses (at least locally) are well above the fracture stress of the NdzFe,,B phase, or above the failure stress of the interphase interface. However, the observation of a predominantly intergranular fracture (Figs 1 and 6) indicates that the second case is more likely. In addition, the results of Auger elemental mapping as well as X-ray microanalysis indicate that the intergranular fracture is likely to be aided by the presence of oxygen in the grain boundary phase. In general, the intrinsic coercivity, iHc, for a nucIeation type magnet can be written as: iHc = c. HA-Nd.M, [26], where, HA is the anisotropy field of the main magnetic phase; c is a constant which depends upon the nature of the nucleation sites; N,, is the demagnetization constant which is dependent upon the shape of the grains and the degree of interaction across grain boundaries and two-phase interfaces; M, is the saturation magnetization. However, the anisotropy field will be strongly influenced by local changes in composition, local lattice strains, especially near grain boundaries and two-phase interfaces. Thus, an effective value of the anisotropy field must be used in the determinatjon of the intrinsic coercivity. These considerations show that two properties, i.e. the effective anisotropy field and the saturation magnetization will influence the value of intrinsic coercivity, for given values of “c” and Nd, If the quenching stresses are sufficient to cause cracking or microcracking of the polycrystalline magnet, then it is possible that the demagnetizing fields at the cracked surfaces will be important. This is because cracking causes the formation of free surfaces, at which free poles will arise. These free poles will lead to high local demagnetizing fields. However, the effect of such a phenomenon appears to be insignificant in the case of water quenched samples. This can be shown from the results of two experiments. In the first, a strong effect of post-quench anneal on the intrinsic coercivity is observed experimentally (as was shown in Fig. 2). Thus, if the sample is annealed at 50&6OO’C for one hour the iHc can be raised to almost that of the slow-cooled sample. However, if quench crack induced demagnetizing fields are the main cause for the drop in iHc, then the annealing treatment should not show an increase in iHc, since many cracks should still remain on annealing. The second evidence which rules out the idea that internal quench cracks induced demagnetizing fields are the cause for the drop in iHc is the results of applied field dependence of iHc. In general, for nucleation controlled magnets, a strong dependence of iHc upon the applied external field is observed [27]. The effective demagnetizing field can be determined from a plot of intrinsic coercivity vs applied field, HdPP.Figure 14 shows iHc vs HaPPfor: (1) the water quenched sample; (2) the water quenched and annealed sample; (3) the slowly cooled sample. There is no significant difference between the three samples in terms of the value of
3146
RAMESH and THOMAS:
IRON-RARE
EARTH-BORON
PERMANENT
MAGNETS
OXYGEN IN FCC PHASE
OXYGEN IN FCC PHASE
OXYGEN IN FCC PHASE
lF~EOF~~()F~i,~Fei28Fek~~F~k2~B~ GRAIN BOUNDARY MATRIX RE2Fe140
PHASE
4JLL
FCC PHASE
INTERFACE
Fig. 13. Schematic representation of a unit cell of the RE,Fe,,B phase, situated at the grain boundary or the two-phase interface of a grain. For x > 0, the environment consists of the oxygen stabilized f.c.c. phase, while for x < 0 it is the RE,Fe,,B phase. An oxygen atom at the interface is shown in order to depict possible interaction with Nd in the matrix phase.
Harp at which iHc begins to rise (the effective demagnetizing field). Thus, the effect of demagnetizing field due to internal quench cracks does not appear to be the dominant cause for the drop in iHc with cooling rate, although quench cracks are observed. It is clear that the effective anisotropy field near the interfaces must be examined in detail. Specifically, the effect of local composition, especially oxygen, and the effect of localized strains, primarily due to quenching, on the magnetocrystalline anisotropy and the effective anisotropy field need further examination. Theoretical and experimental estimations of these effects are currently under way.
the differently treated magnets, it is shown that demagnetization fields at quench cracks are not the main cause for the difference in intrinsic coercivity for the different cooling rates. A qualitative argument for the influence of oxygen in reducing the magnetocrystalline anisotropy field of the RE,Fe,,B phase has been suggested to explain the low value of
CONCLUSIONS
In conclusion it has been shown that post-sintering cooling rates have a definite influence on the intrinsic coercivity and energy product of Iron-Rare Earth-Boron magnets. After water quenching, the intrinsic coercivity was found to be lower than in a slowly cooled magnet. Concurrently, water quenched samples contained many intergranular quench that quenching stresses are cracks, indicating significant. No specific differences in microanalytical details of the rare earth elements in the matrix and grain boundary phase and their interface were observed for the different cooling rate samples. From the similar field dependence of intrinsic coercivity in
0 0
10
Applied
20
field,Hm,
(kG)
Fig. 14. Applied field dependence of the intrinsic coercivity for (1) water quenched samples; (2) water quenched + annealed samples; (3) slowly cooled samples.
RAMESH and THOMAS:
IRON-RARE EARTH-BORON PERMANENT MAGNETS
intrinsic coercivity compared to the anisotropy field, even in a slowly cooled sample. Finally, it is possible that local strains at the grain boundaries and twophase interfaces could cause a large reduction in the magnetocrystalline anisotropy of the RE,Fe,,B phase, thus leading to an even lower intrinsic coercivity in the rapidly quenched magnet. Ackn~wledgeme~f.~-The
authors wish to thank Professor
M. Okada, Tohuku University, Japan for kindty providing the alloys used in this study. The authors also wish to acknowledge Professor W. A. Soffa for stimulating discussions on this subject. This work was supported by the Director, Office of Basic Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under contract No. DE-AC0376SF00098. REFERENCES 1. J. F. Herbst, J. 3. Croat, F. E.(1984). Pinkerton and W. B. Yelon, Phys. Rev. B. 29, 4176 2. M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura and K. Hiraga, IEEE Trans. h4ag. Mag-20,1584 (1984). 3. See for example Proc. Znfermag 1984, 1985, 1986 and 1987; Proc. Annuaf EMU
Cor$
9. R. Ramesh, J. K. Chen and G. Thomas, J. appl. Phys. 61, 2993 (1987). 10. R. K. Mishra, J. K. Chen and G. Thomas, J. appl. Phys. 59, 2244 (1986).
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